1. Field of the Invention
The invention relates to a door handle notably for a motor vehicle, and more particularly a door handle incorporating at least one presence sensor to detect the presence of a user at the handle.
The invention also relates to a hands-free access system for a motor vehicle including such a handle.
2. Description of the Related Art
In the early days of the automobile, the door handle served only to transmit mechanical movements via tie-rods to a door-catch to open the door. Today the door handle has undergone great changes.
In particular, in so-called “hands-free” systems enabling locking and unlocking of a motor vehicle without a mechanical key or remote control, the handle has become a special interface between the user, wearing an identifier in the form of a badge, for example, and the vehicle's onboard system.
A handle as used in a “hands-free” system is shown in FIG. 1.
Generally, such a handle 1 comprises two parts, a grasping part 2 that is mobile relative to the door 4 of the vehicle and a fixed part 3 that is essentially decorative or houses, for example, a backup lock. Both the mobile part 2 and the fixed part 3 are made of plastic material.
As seen in FIG. 1, the mobile part has an internal cavity 6 serving as a housing for a support module 7. To enable the module to be fitted in the handle the internal cavity 6 (hereinafter referred to as the “housing”) is open on one side in a zone located opposite the fixed part 3. The housing 6 is closed by a plugging part 6bis. 
The support module 7 includes presence sensors and notably an approach sensor 9 and a tactile sensor 8.
The approach sensor 9 is used to initiate the communication between the identifier and the vehicle's onboard system when the user approaches the vehicle, whereas the tactile sensor 8 is used to detect a voluntary action by the user to lock the vehicle.
As shown in FIG. 1, the approach detection zone 12 associated with the approach sensor 9 is situated between the door 4 and the grasping part 2 of the handle.
The approach sensor 9 is for example a capacitive sensor that operates by measuring the variation of the electromagnetic field surrounding it. It has at least one detection electrode 10 whose shape enables an extended and well-defined detection zone between the door 4 and the grasping part of the handle 2.
The tactile sensor 8 is also a capacitive sensor, for example. It has a detection electrode 10 which enables detection over a precisely defined zone 11 located at the outer surface of the handle. This tactile detection 11 zone has high sensitivity to a touching action.
Such an arrangement of the various parts in the handle has the disadvantage of offering very limited freedom for positioning the various sensors on the support module owing to the small size of the housing 6, imposed by mechanical constraints. The result is that the location of the various detection zones is practically imposed by the geometry of the handle and its housing.
Consequently, these various detection zones may find themselves located in relatively inaccessible places or may present major usability problems. Furthermore, such an arrangement does not enable the use of standard handles usable by both left-handed and right-handed users, since this implies being able to choose the position of the various detection zones such that they are quite distinct and easily accessible to the user.
Moreover, it has been observed, later, that when such handles are painted, metal-plated or even solid metal, the operation of the presence sensors is highly perturbed.
In effect, a conductive coating, which can be a chemical deposit such as chrome-plating, paint, a primer for decorative coatings or even the material constituting the handle, causes modification of the capacitive couplings between the detection electrodes of the various sensors and the surface of the handle.
This perturbation mainly associated with the electrical conductivity of the coating results in a change of the shape of the presence detection zones covered by the various presence sensors and consequently lowers sensitivity of the sensor.
In the case of the approach sensor, spreading of the detection zone is observed, accompanied by a substantial reduction in the detection distance from the handle. Remote detection (i.e. at a distance of a few centimeters) of an approaching user is no longer possible: the user must be within one centimeter—or even in contact with the handle—for the detection to be made. The reason is that the conductive coating constitutes a screen for the electric field lines of the approach sensor, which greatly reduces the ability to detect presence by measuring electrical capacitance.
In the case of the tactile sensor, spreading of the tactile detection zone over a large part of the handle surface is usually observed, with a consequent large drop in sensitivity of the sensor and total loss of the tactile detection function.
To illustrate this problem better, FIG. 2 shows schematically the sensitivity S of a tactile sensor 8 relative to the width of the electrode 10, in one dimension only.
The solid curve A shows the sensitivity curve of the tactile sensor for a handle made of electrically insulating material. This curve A has a flat section P whose width E corresponds substantially to the width of the electrode 10. It is seen that the sensitivity drops off strongly on each side.
The dashed curve B shows the sensitivity of the tactile sensor for a handle with a conductive surface coating. This sensitivity curve is broader and its maximum height is much less than curve A, which implies malfunctions of the sensor 8; moreover, this curve has no characteristic plateau, which signifies that the detection zone is spread and not very well defined, which is a serious handicap in terms of usability.
As stated previously, the presence of this conductive coating perturbs the tactile sensor due to the spreading of the tactile detection zone over a larger surface of the handle, resulting in serious loss of sensitivity of the sensor. The tactile detection is therefore strongly perturbed.
The lack of flexibility in the positioning of the detection zones of the approach and tactile sensors plus, in the case of painted or metal-plated handles, the modification of these zones and the resulting weak detection are particularly problematic.